title: Blockade of adrenomedullin receptors reverses morphine tolerance and its neurochemical mechanisms.
abstract: Adrenomedullin (AM) has been demonstrated to be involved in the development of opioid tolerance. The present study further investigated the role of AM in the maintenance of morphine tolerance, morphine-associated hyperalgesia and its cellular mechanisms. Intrathecal (i.t.) injection of morphine for 6 days induced a decline of its analgesic effect and hyperalgesia. Acute administration of the AM receptor antagonist AM(22-52) resumed the potency of morphine in a dose-dependent manner (12, 35.8 and 71.5 μg, i.t.). The AM(22-52) treatment also suppressed morphine tolerance-associated hyperalgesia. Furthermore, i.t. administration of AM(22-52) at a dose of 35.8 μg reversed the morphine induced-enhancement of nNOS (neuronal nitric oxide synthase) and CGRP immunoreactivity in the spinal dorsal horn and/or dorsal root ganglia (DRG). Interestingly, chronic administration of morphine reduced the expression of the endogenous opioid peptide bovine adrenal medulla 22 (BAM22) in small- and medium-sized neurons in DRG and this reduction was partially reversed by the administration of AM(22-52) (35.8 μg). These results suggest that the activation of AM receptors was involved in the maintenance of morphine tolerance mediating by not only upregulation of the pronociceptive mediators, nNOS and CGRP but also the down-regulation of pain-inhibiting molecule BAM22. Our data support the hypothesis that the level of both pronociceptive mediators and endogenous pain-inhibiting molecules has an impact on the potency of morphine analgesia. Targeting AM receptors is a promising approach to maintain the potency of morphine analgesia during chronic use of this drug.
authors.affiliation: Provincial Key Laboratory of Developmental Biology and Neuroscience, College of Life Sciences, Fujian Normal University, Fuzhou, Fujian 350108, People's Republic of China.
authors.forename: Dongmei
authors.initials: D
authors.lastname: Wang
authors.forename: Peiwen
authors.initials: P
authors.lastname: Chen
authors.forename: Qi
authors.initials: Q
authors.lastname: Li
authors.forename: Rémi
authors.initials: R
authors.lastname: Quirion
authors.forename: Yanguo
authors.initials: Y
authors.lastname: Hong
mesh_terms: D053607:Adrenomedullin / Q000008:administration & dosage / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004361:Drug Tolerance / Q000502:physiology*; D004744:Enkephalin, Methionine / Q000031:analogs & derivatives / Q000096:biosynthesis; D006930:Hyperalgesia / Q000188:drug therapy / Q000503:physiopathology; D007278:Injections, Spinal; D008297:Male; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D052248:Nitric Oxide Synthase Type I / Q000378:metabolism; D010147:Pain Measurement / Q000379:methods; D010446:Peptide Fragments / Q000008:administration & dosage / Q000494:pharmacology*; D020671:Posterior Horn Cells / Q000378:metabolism; D011498:Protein Precursors / Q000096:biosynthesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D058265:Receptors, Adrenomedullin / Q000037:antagonists & inhibitors / Q000502:physiology*; D018015:Receptors, Calcitonin Gene-Related Peptide / Q000096:biosynthesis
chemical_list: D010446:Peptide Fragments; D011498:Protein Precursors; D058265:Receptors, Adrenomedullin; D018015:Receptors, Calcitonin Gene-Related Peptide; C104782:adrenomedullin (22-52); D053607:Adrenomedullin; D004744:Enkephalin, Methionine; C028837:BAM 22P; D009020:Morphine; D052248:Nitric Oxide Synthase Type I